<p> Chapter 13: Ceramics and Properties of Ceramics</p><p>Root: keramikos (Greek): “burnt stuff”.</p><p>Traditional Ceramics: china, porcelain, bricks, tiles, glasses and high temp. ceramics</p><p>New generation ceramics: (Since better understanding of the fundamental character of ceramics, and parameters influencing their unique properties)</p><p>Impact: electronic, computer, communication, aerospace and other industries.</p><p>CERAMIC STRUCTURES: composed of electrically charged ions; not atoms. Partial or total ionic bonding.</p><p>Cations: metallic ions, positively charged, since they have given up valence electrons to non- metallic ions. Anions: non-metallic ions, negatively charged</p><p>CRYSTAL STRUCTURES: Influenced by two characteristics: 1) Magnitude of the electrical charge on each of the component ions. The crystal must be electrically neutral. ie., balancing of positive and negative charges.</p><p>2) Relative sizes of Cations and Anions. Ionic radii: rccation and raanion</p><p>Since they give up electrons, Cations are usually smaller than Anions. : rc/ra <1 Each ion desires to have as many nearest neighbours of the opposite charge. Stable when anions are touching a cation.</p><p>Coordination No: number of anion nearest neighbours of a cation. P. 378-379 Table 13.1: Table 13.2 Ionic radii listed</p><p>Problem 13.1 Show that the minimum cation to anion radius ratio for the coordination number 3 is 0.155.</p><p>AP=rA</p><p>AO= rA + rC cos (30o)=AP/AO= = Types of Structures AX-Type Crystal Structure  Equal number of cations and anions</p><p> Several different crystal structures for AX compounds</p><p>1) AX-Rock Salt Structure: NaCl/Rock salt  Coordination number for anions and cations: 6</p><p> cation/anion ratio: 0.414-0.732</p><p>FCC arrangement of anions Two interpenetrating FCC lattices FCC arrangement of cations </p><p>Eg. NaCl, MgO, MnS, LiF and FeO 2) AX - Cesium Chloride Structure: CsCl  Coordination number for both ions: 8</p><p> Anions at centres of cube</p><p> Cation at body (cube-centre)</p><p>3) AX – Zinc Blende Structure: ZnS/sphalerite/Zinc Blende  Coordination number: 4</p><p> S atoms: at cube corners and face positions</p><p> Zn atoms: interior tetrahedral positions, Each Zn atom is bonded to four S atoms and vice versa. Often highly covalent. eg. ZnS, ZnTe, SiC</p><p>4) AmXp Type Crystal Structures</p><p> m and/or p ≠ 1</p><p> eg. AX2: CaF2/Flourite</p><p> Coordination Number: 8 rc/ra = 0.8</p><p>2+ +  Similar to CsCl structure. But, only half as many Ca as F ions so, only half the centres cube positions are occupied by Ca2+ ions.</p><p> Eg., UO2, PuO2 and ThO2</p><p>5) AmBnXp Type Crystal Structure ("Two types of cations”)</p><p> eg., BaTiO3/Barium Titanate</p><p>Ceramic Density Calculations p.36 Where: n=number of atoms associated with each unit cell A= atomic weight Vc = Volume of the unit cell Na = Avagadro’s number (6.023 x 10^23 atoms /mole)</p><p>Where: n' = the number of formula units within the unit cell (eg.BaTiO3: 1, 2 ,3)</p><p>AC = the sum of the atomic weights of all cations in the formula unit</p><p>AA = the sum of the atomic weights of all the anions in the formula unit</p><p>VC = the unit cell volume</p><p>NA = Avagadro’s number</p><p>Compute the theoretical density of NaCl on the basis of crystal structure. How does this compare with its measured density? n' = 4/unit cell</p><p>AC = ANa = 22.99 g/mol</p><p>AA = Acl = 35.45 g/mol</p><p>3 Vc = a a = 2(rNa + rcl) rNa= 0.102 nm rcl =0.181nm =2.14 g/cm3 (Expected value: 2.16 g/cm3) Ceramic Phase Diagrams</p><p>The Al2O3-Cr2O3 System Isomorphous (density similar to Cu-Ni)</p><p>3+ 3+  Fig. 13.24: Al2O3-Cr2O3 Substitution Solid Solution. (Al substitutes for Cr ), same charge.</p><p> Similar radii: 0.053nm (Al3+) 0.063 nm (Cr3+)</p><p> Same crystal structure</p><p>The MgO-Al2O3 System (similar to Pb-Mg)</p><p> Intermediate phase: MgAl2O4 spinel (ie., MgO-Al2O3)</p><p> Fig.13.25: Distinct compound, single phase field (not a line as for Mg2Pb)</p><p> Spinel nonstoichiometric (range of compositions)</p><p> Differences in charge: Mg2+ Al3+</p><p>0.072 nm (Mg2+) 0.053 nm (Al3+)</p><p> MgO is virtually insoluble in Al2O3; so no terminal solid solution at RHS</p><p> Two eutectics</p><p> Constant melting of spinel at 2100°C</p><p>The ZrO2-CaO System (zirconia/calcium)  Three reactions: eutectic, pertectic and eutectoid.</p><p> Three different crystal structures: tetragonal, monoclinic, cubic</p><p> Pure ZrO2: tetragonal to monoclinic at 1000°C</p><p> a= b ≠ c a ≠ b ≠ c α = β = γ = 90° α = γ = 90° ≠ β</p><p>Results in cracking so, “stabilize” by adding 3 to 7 wt.% CaO to ZrO2. This eliminates cracking, as no such phase change occurs.</p><p> p.402 The SiO2-Al2O3 System</p><p> SiO2 and AL2O3: Not mutually soluble</p><p> No terminal solid solutions at both extremities</p><p> Si02: cristobalite (most stable form)</p><p> 3Al2O3-2SiO2: Mullite</p><p> Simple eutectic at 1587 oC p.407 Plastic Deformation: Crystalline Ceramics:  Why are crystalline ceramics hard and brittle?</p><p> Few slip systems for the movement of dislocations (due to the electrically charged nature of ions).</p><p> Electrostatic repulsion of like charges eliminates several possibilities of slip (disloc. motion)</p><p> Deformation by motion of dislocations</p><p> Hard and brittle :difficulty in dislocation motion (slip)</p><p> Bonding: ionic; therefore few slip systems (planes and directions) for dislocation motion, due to electrically charged nature of ion.</p><p> For slip in some direction, ions are brought close to one and other; but this type of motion is restricted due to electrostatic repulsion</p><p> For ceramics with covalent bonding, slip is difficult because:</p><p>1. Covalent bonds are strong</p><p>2. Limited no. of slip systems</p><p>3. Dislocation structures are complex p.407 Noncrystalline Ceramics (glassy)  Deformation is not by dislocation motion</p><p> But by viscous flow</p><p> Rate of deformation  applied stress</p><p> Atoms/ions slide past one another by breaking/reforming bonds</p><p> Viscosity: measure of resistance to deformation</p><p>Where: η, viscosity. Poises (P) pascal-second τ, Applied shear stress dv, change in viscosity dy, distance in direction perpendicular to and away from plates.</p><p>1Poise 1 P=1 dyne-second/cm2 1Pa-s=1N-sec/m2 10 P=1 Pa-s Water: 10-3 Pa-s Glass: very large (since strong interatomic bonding). As temperature increases, the magnitude of bonding is diminished, sliding motion or flow of atoms or ions is facilitated, then there is a decrease in viscosity.</p>